Method for cancelling a data transmission of a neighboring cell

ABSTRACT

A method of interference cancellation is proposed. A UE obtains configuration information of a data transmission from a neighboring cell via an interference channel in a mobile communication network. The UE receives radio signals on a set of data resource elements as determined based on the obtained configuration information. The UE then estimates the interference channel corresponding to the data transmission from the neighboring cell based on the received radio signals on the set of data resource elements. Finally, the UE cancels the data transmission from the neighboring cell based on the estimated interference channel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 from U.S.Provisional Application No. 61/932,827, entitled “Methods for Cancellinga Data Transmission of a Neighboring Cell,” filed on Jan. 29, 2014, thesubject matter of which is incorporated herein by reference.

TECHNICAL FIELD

The disclosed embodiments relate generally to mobile communicationnetworks, and, more particularly, to methods for interferencecancellation of data transmission from neighboring cells.

BACKGROUND

Long Term Evolution (LTE) is an improved universal mobiletelecommunication system (UMTS) that provides higher data rate, lowerlatency and improved system capacity. In LTE systems, an evolveduniversal terrestrial radio access network includes a plurality of basestations, referred as evolved Node-Bs (eNBs), communicating with aplurality of mobile stations, referred as user equipment (UE). A UE maycommunicate with a base station or an eNB via the downlink and uplink.The downlink (DL) refers to the communication from the base station tothe UE. The uplink (UL) refers to the communication from the UE to thebase station. LTE is commonly marketed as 4G LTE, and the LTE standardis developed by 3GPP.

In cellular networks, the inter-cell interference is commonly seen at aUE when a “desired” data transmission (i.e., one from the “servingcell”) is interfered by an interfering data transmission from aneighboring cell to another UE that has the neighboring cell as its“serving cell”. When the network deployment is synchronized among allcells with sufficient accuracy (e.g., to a GPS signal), the mobilereceiver may attempt to cancel the interference in order to achieve abetter throughput on the desired data transmission.

Starting from April 2013, 3GPP started a new study item (SI), “NetworkAssisted Interference Cancellation and Suppression” (NAICS), toinvestigate the benefit on system throughput by leveraging receiver'scapability of interference cancellation (IC). There are many methods forinterference cancellation at the receiver, but typically, they allexploit some known or estimated characteristics of the interference datatransmission such as the corresponding interference channel, themodulation order of the interference symbols, the coding information topossibly reconstruct the interference signal, and so on. Compared withinterference-suppression receivers, IC-receivers usually need moretransmission parameters of interference.

Commonly investigated IC techniques in literature may includesymbol-level based IC (SLIC) and codeword-level IC (CWIC). SLIC is an ICtechnique that detects interfering signal, which is supposed to befinite-constellation modulated, in a per-symbol basis. CWIC is atechnique in which a receiver decodes and re-encodes an interferencecodeword to reconstruct the contribution of the interference signal onits received signal. Compared to SLIC, a receiver needs more informationon interference to access CWIC, such as the modulation and coding scheme(MCS) index and the rule for scrambling the bit stream of interference.Obtaining the interference characteristics, such as the modulation orderor encoding rules of the interfering signal, is important for all ICtechniques. The characteristics could be either blindly detected byvictim receiver or informed from network side.

The challenge of interference cancellation lies on the fact that datatransmission can be very dynamic in a neighboring cell due to thescheduling behavior of a base station when serving multiple UEs at thesame time. As a result, interference may or may not be present from timeto time depending on the traffic loading; different UEs may be scheduledat different time; the frequency resources allocated to a UE in an OFDMAbased system (e.g., LTE) change from time to time; the modulation orderand/or coding rate change according to the dynamic channel condition;and so on.

There are study results that showed promising gain assuming known orreliable detection of some transmission parameters of the interferingdata transmission. However, the so-called “blind-detection” receiver canbe very complex and unreliable if it must detect or estimate all thecharacteristics of a possible interfering data transmission, especiallywhen the resources allocated to a data transmission can be very dynamicin both the time and frequency dimension as in LTE. Given thesignificant throughput gain from interference cancellation, especiallyfor OFDMA-based networks such as LTE, there is a need to enable robustcancellation of neighboring cell's data transmission.

SUMMARY

A method of interference cancellation is proposed. A UE obtainsconfiguration information of a data transmission in a mobilecommunication network. The data transmission is transmitted from aneighboring cell to the UE via an interference channel. The UE receivesradio signals on a set of data resource elements as determined based onthe obtained configuration information. The UE then estimates theinterference channel corresponding to the data transmission from theneighboring cell based on the received radio signals on the set of dataresource elements. Finally, the UE cancels the data transmission fromthe neighboring cell based on the estimated interference channel.

In one embodiment, the configuration information comprises a resourceallocation type of the data transmission from the neighboring cell. Inanother embodiment, the configuration information comprises a basicresource allocation unit of the data transmission from the neighboringcell. The UE may determine the location of the set of date resourceelements from the resource allocation information and then estimate theinterference channel accordingly. In one example, the UE first estimatesa desired signal corresponding to a reference signal transmitted fromthe serving cell and then subtracts from the received signals theestimated desired signal to estimate the interference channel.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a mobile communication network with interferencecancellation of data transmission from neighboring cells in accordancewith one novel aspect.

FIG. 2 is a simplified block diagram of a base station and a userequipment that carry out certain embodiments of the present invention.

FIG. 3 illustrates physical resource blocks (PRBs) and resourceallocation in LTE.

FIG. 4 illustrates mapping from virtual resource block to physicalresource block for DVRB.

FIG. 5 illustrates functional blocks in a communication system that mapsinformation bits of a transport block to codewords and then maps tobaseband signals for transmission.

FIG. 6 illustrates cell-specific reference signals in a PRB pair for twoantenna ports.

FIG. 7 illustrates channel estimation of an interference channel.

FIG. 8 illustrates one embodiment of cancelling data transmission from aneighbor cell.

FIG. 9 is a flow chart of a method of interference cancellation from UEperspective in accordance with one novel aspect.

FIG. 10 is a flow chart of one embodiment of a method of interferencecancellation from eNB perspective in accordance with one novel aspect.

FIG. 11 is a flow chart of another embodiment of a method ofinterference cancellation from eNB perspective in accordance with onenovel aspect.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 illustrates a mobile communication network 100 with interferencecancellation of data transmission from neighboring cells in accordancewith one novel aspect. Mobile communication network 100 is an OFDMnetwork comprising a plurality of user equipments UE 101, UE 102, and UE103, a serving base station eNB 104, and a neighbor base station eNB105. In the example of FIG. 1, UE 101 is served by its serving basestation eNB 104. UE 101 receives desired radio signal 111 transmittedfrom eNB 104. However, UE 101 also receives interfering radio signals.In one example, UE 101 receives intra-cell interfering radio signal 112transmitted from the same serving eNB 104. Typically, such intra-cellinterference is due to multi-user multiple-input multiple-output(MU-MIMO) transmission intended for other UEs (e.g., UE 102 and UE 103)in the same serving cell. In another example, UE 101 receives inter-cellinterfering radio signal 113 transmitted from neighbor base station eNB105. UE 101 may be equipped with an interference cancellation (IC)receiver that is capable of cancelling the contribution of theinterfering signals from the desired signals.

The characteristics of a data transmission in LTE downlink illustratethe dynamic nature given the resources can be allocated in both time andfrequency domain. Here, a data transmission refers a data-bearingPhysical Downlink Shared Channel (PDSCH) in LTE. The data carried inPDSCH is known as transport block (TB) which corresponds to a MAC-layerPDU (Protocol Data Unit). PDSCH transmission is characterized bytransmission parameters in three general categories: 1) Resourceallocation of PDSCH; 2) PDSCH signal structure, i.e., mapping of a TB tosignal according to a transmission mode (TM); and 3) Reference signals,to allow estimation of the corresponding channel for PDSCH demodulationto decode out the TB. These characteristics of data transmission areimportant for reliable interference cancellation.

In accordance with one novel aspect, UE 101 receives configurationinformation from serving eNodeB 104 via higher layer signaling. Theconfiguration information comprises resource allocation (RA)information, transmission mode, and other parameters related to theinterfering data transmission from the neighbor eNodeB 105. UE 101 thendetermines a set of data resource elements from the configurationinformation. UE 101 then estimates the interference channelcorresponding to the data transmission of the neighbor cell based on thereceived signal on the set of data resource elements. Finally, UE 101cancels the data transmission from the neighboring cell based on theestimated interference channel.

FIG. 2 is a simplified block diagram of a base station 201 and a userequipment 211 that carry out certain embodiments of the presentinvention in a mobile communication network 200. For base station 201,antenna 221 transmits and receives radio signals. RF transceiver module208, coupled with the antenna, receives RF signals from the antenna,converts them to baseband signals and sends them to processor 203. RFtransceiver 208 also converts received baseband signals from theprocessor, converts them to RF signals, and sends out to antenna 221.Processor 203 processes the received baseband signals and invokesdifferent functional modules to perform features in base station 201.Memory 202 stores program instructions and data 209 to control theoperations of the base station. Similar configuration exists in UE 211where antenna 231 transmits and receives RF signals. RF transceivermodule 218, coupled with the antenna, receives RF signals from theantenna, converts them to baseband signals and sends them to processor213. The RF transceiver 218 also converts received baseband signals fromthe processor, converts them to RF signals, and sends out to antenna231. Processor 213 processes the received baseband signals and invokesdifferent functional modules to perform features in UE 211. Memory 212stores program instructions and data 219 to control the operations ofthe UE.

Base station 201 and UE 211 also include several functional modules tocarry out some embodiments of the present invention. The differentfunctional modules can be configured and implemented by software,firmware, hardware, or any combination thereof. The function modules,when executed by the processors 203 and 213 (e.g., via executing programcodes 209 and 219), for example, allow base station 201 to schedule (viascheduler 204), encode (via encoder 205), mapping (via mapping module206), and transmit control information and data (via control module 207)to UE 211, and allow UE 211 to receive, de-mapping (via de-mapper 216),and decode (via decoder 215) the control information and data (viacontrol module 217) accordingly with interference cancellationcapability. In one example, base station 201 provides configurationinformation that include parameters related to data transmission fromneighboring cells to UE 211. Upon receiving the related parameters, UE211 is then able to estimate the interference channel and then performinterference cancellation via IC module 214 to cancel the datatransmission from the neighboring cells accordingly.

Resource Allocation of PDSCH

FIG. 3 illustrates physical resource blocks (PRBs) and resourceallocation in 3GPP LTE. In a 3GPP LTE system based on OFDMA downlink,the radio resource is partitioned into subframes in the time domain.Each subframe is comprised of two slots, and each slot has seven OFDMAsymbols in the case of normal Cyclic Prefix (CP), or six OFDMA symbolsin the case of extended CP. Each OFDMA symbol further consists of anumber of OFDMA subcarriers in the frequency domain depending on thesystem bandwidth. The basic unit of the resource grid is called aResource Element (RE), which spans an OFDMA subcarrier in frequencydomain over one OFDMA symbol in time domain. Each RE carries a datasymbol from the QPSK/16QAM/64QAM modulation constellation. Resourceelements are grouped into resource blocks, where each resource block(RB) consists of 12 consecutive subcarriers in one slot, whichconstitutes 84 REs in normal CP and 72 REs in extended CP. The minimalgranularity of resource allocation in LTE is a PRB. PRBs are indexedsequentially in frequency domain.

Since the base station may allocate in PDSCH a contiguous ornon-contiguous set of PRBs to a user equipment or UE, a virtual RB (VRB)is defined to allow more efficient indexing in resource allocation. AVRB of localized type or LVRB is mapped directly to a PRB with the sameindexing. When a VRB index is assigned to a UE, both PRBs locating inthe same frequency location in the first and second slots of a subframeare allocated. On the other hand, a VRB of distributed type (DVRB) mapsto two PRBs at different frequency locations in the two slots, accordingto system bandwidth and a value N_(gap).

FIG. 4 illustrates mapping from virtual resource block to physicalresource block for DVRB. Conceptually the mapping from VRB to PRB fortwo slots consists of a first interleaving step and a second hoppingstep, as illustrated in FIG. 4. RB pair interleaving is first performedfor the mapping from VRB to PRB, for the first slot in one subframe.This interleaving rule is uniquely determined by the value of systembandwidth. Then a second step is applied to determine the exact locationof the physical resource-block for the second slot by additionallyhopping the PRB-index by N_(gap). For the case with bandwidth less than10 MHz, the value of N_(gap) is uniquely determined by bandwidth. If thebandwidth is larger than or equal to 10 MHz, N_(gap) will have twocandidate values and the selection is signaled dynamically on thescheduling assignment.

In LTE, the resource allocation (RA) is signaled via a control channelcalled PDCCH (Physical Downlink Control Channel). A UE needs to decodePDCCH in order to demodulate PDSCH to decode the TB on asubframe-by-subframe basis. However, for the interference PDSCH, it isimpractical for the victim UE to decode the interference PDCCH due tocomplexity and the fact that interference PDCCH targets at a differentUE than the victim UE. Therefore, without the RA information of theinterference PDSCH, a UE must estimate the characteristics of theinterference on a PRB level (for DVRB) or a PRB pair level (for LVRB).

PDSCH Signal Structure

FIG. 5 illustrates functional blocks of a transmitting device in acommunication system that map information bits of a transport block (TB)to codewords and then map to baseband signals for transmission. In step501, the information bits are arranged into transport blocks (TBs) andattached with CRC. In addition, the TBs are segmented into code blocksand attached with CRC. In step 502, channel coding (forward errorcorrection such as Turbo coding) is performed with certain code rate. Instep 503, rate matching is performed, which creates an output with adesired code rate, and where the TBs are mapped into codewords. In step504, the codewords are scrambled based on predefined scrambling rule(e.g., scramble with a corresponding Radio Network Temporary Identifier(RNTI) of the UE). In step 505, modulation mapping is performed, wherethe codewords are modulated based on various modulation orders (e.g.,QPSK, QAM) to create complex-valued modulation symbols. In step 506,layer mapping is performed, where the complex-valued symbols are mappedonto different MIMO layers depending on the number of transmit antennaused. In step 507, precoding is performed with certain precoding matrixindex (PMI) for each antenna port. In step 508, the complex-valuedsymbols for each antenna are mapped onto corresponding resource elements(REs) of physical resource blocks (PRBs). Finally, in step 509, OFDMsignals are generated for baseband signal transmission via antennaports.

The mapping rules in these functional blocks should be known for areceiving device to receive the transport blocks. A UE receivesinformation-bearing signal propagating though wire channel or wirelesschannel and processes it to recover the transport block. For the UE toreceive TBs carried by PDSCH, it first needs to know the DCI carried byPDCCH associated with these transport blocks. The DCI indicates therules that map the information bits of each TB to the modulated symbolscarried on PDSCH, the RB-allocation for the encoded and modulatedsymbols of the transport blocks, information related to the referencesignals used for channel estimation, and power control commands. UEdecodes the TBs based on received control information and the configuredparameters provided by network.

In addition, several transmission formats are allowed for PDSCH in LTE.Each transmission format is referred to as a “transmission mode” (TM)that is mainly characterized by the different precoding processing andthe number of layers (i.e., rank) associated with precoding. TM of thePDSCH is configured by eNodeB via high-layer messages, but precoding andrank are dynamically signaled in PDCCH. Again, it is impractical for avictim UE to decode the interference PDCCH due to complexity and thefact that interference PDCCH targets at a different UE than the victimUE.

Reference Signals

To allow a UE to demodulate PDSCH, a UE needs to derive the effectivechannel corresponding to the PDSCH. LTE defines two types of referencesignals (RS) for that purpose. Cell-specific reference signals (CRS) areutilized by UEs for the demodulation of control/data channels innon-precoded or codebook-based precoded transmission modes, radio linkmonitoring and measurements of channel state information (CSI) feedback.All UEs in the serving cell can use CRS to estimate the channelassociated with 1 or 2 or 4 antenna ports of the serving eNB.UE-specific Demodulation RS (DM-RS) are utilized by UEs for thedemodulation of control/data channels in non-codebook-based precodedtransmission modes. DMRS are used by a specific UE because they aretransmitted by the same precoding as that applied to data REs, and thuscan be used to derive the effective channel. Note that unlike CRS thatare always present regardless of the presence of PDSCH or not, DMRS arenot transmitted in PRBs when there is no PDSCH in that PRB.

FIG. 6 illustrates one example of cell-specific reference signal in aPRB pair for 2 antenna ports (denoted as R0 and R1). After obtaining thechannel corresponding to each antenna ports from relevant CRS, the UEcan apply the dynamically signaled precoding information calledPrecoding Matrix Index (PMI) to derive the effective channel associatedwith the precoded PDSCH transmission (one channel for each layer orrank).

LTE defines TMs in association with CRS or DMRS as follows: 1) TM 1:single antenna port (i.e., single CRS port); 2) TM2/3: transmissiondiversity (T×D) for rank-1 and large-delay CDD for rank-2 (2 or 4 CRSports); 3) TM4/6: rank-1 or 2 precoding based on 2/4 CRS ports; and 4)TM8/9/10: rank-1 or 2 precoding based on DMRS ports. Even though TM isconfigured via high layer messages to each UE specifically, the PMI andRI (rank indicator) for TM4/6 are dynamically signaled in PDCCH.

Interference Cancellation Method

To cancel the interference PDSCH from a neighboring cell, the UE needsthe knowledge of the interference channel corresponding to theinterference PDSCH. Once the interference channel is estimated, the UEcan apply different interference cancellation methods. For example,applying a linear MMSE receiver to linearly suppress the interference oneach data RE. If the UE can further detect the modulation order of thePDSCH out of the three constellations (i.e., QPSK, 16QAM, 64QAM), it mayestimate the interference symbol and then try to successively cancel theinterference signal by reconstructing the signal from the estimatedchannel and the estimated symbols (i.e., symbol-level IC).Alternatively, the UE may jointly detect the desired and interferencePDSCH via, for example, a Maximum Likelihood (ML) type of receiverprocessing. Furthermore, if the exact resource allocation (RA) ofinterference PDSCH is known, and along with coding information, theentire PDSCH may be more reliably reconstructed for IC.

Obtaining channel state information relies on the received referencesignals and may need the aid of signaling to know the used precoder. ForPDSCH transmission, the transmission mode could be either CRS-based orDMRS-based. For CRS-based transmission mode, the precoder used by PDSCHis signaled to receiver through control channel. The precoderinformation cannot be extracted from the received cell-specificreference signals. The UE needs to know the PMI that is to be applied onthe channel estimated from the neighboring cell's CRS. In addition, theUE needs to first detect the TM and the presence or absence ofinterference PDSCH in each PRB. On the other hand, for DMRS-basedtransmission mode, precoding is also applied on DMRS. Receiver directlyestimates the compound channel formed by propagation channel and theused precoding vector/matrix for further processing. The UE can estimatethe interference channel and detect its presence directly from DMRS aslong as the DMRS sequence is known.

While the victim receiver is supposed to know the complete controlinformation for its own PDSCH signal through RRC and PDCCH from itsserving eNB, such information associated with interference signal aregenerally unknown to the victim receiver. It is impractical for a UE todetect interference PDCCH to obtain all the transmission parameterscorresponding to the interference PDSCH. An efficient way to signalinformation about the interference signal would reduce the receiver'scomplexity to estimate/detect interference information and help thereceiver to provide better performance resulting from IC gain.

In the “Network Assisted Interference Cancellation and Suppression”(NAICS) study item, various parameter candidates helpful forinterference cancellation were identified. For example, parameters thatare higher-layer configured per the current specifications (e.g.,transmission mode, cell ID, MBSFN subframes, CRS antenna ports, P_(A),P_(B)); parameters that are dynamically signaled per the currentspecifications (e.g., CFI, PMI, RI, MCS, resource allocation, DMRSports, n_(ID) ^(DMRS) used in TM10); and other deployment relatedparameters (e.g., synchronization, CP, subframe/slot alignment).Although it is possible to let receiver detect or estimate theseparameters associated with the interfering signal without any aid ofsignaling, the complexity cost could be very huge to estimate them.Furthermore, since interference characteristic may change for everyPRB/subframe, dynamic signaling all the parameters is not feasible.

Higher Layer Signaling

In LTE Rel-11, it is already made possible to signal, via radio resourcecontrol (RRC) message, a list of neighboring cell IDs, CRS ports, andMBSFN subframe patterns (since CRS is only present on the first symbolof any MBSFN subframe).

In one embodiment, a UE receives the following information about aneighboring cell: 1) Whether the cell can be considered as synchronizedwith the serving cell (including slot alignment); 2) the CP length; and3) the System bandwidth. Once the UE knows a synchronized deployment,the UE can easily decide whether to apply IC receiver or not. Systembandwidth information will be useful to determine the CRS to be used andsome other RA-related knowledge discussed later. Even though the aboveinformation can be obtained once the UE can successfully process thePSS/SSS to learn about synchronization status and CP length, and processPBCH to learn about the system bandwidth, it is noted that theinterference PSS/SSS and PBCH may be subject to stronger desired celltransmission and thus prone to detection error.

In another embodiment, a UE obtains, from a serving cell viahigher-layer messages, configuration information of a data transmissionfrom a neighboring cell, wherein the configuration information includesa specific TM or a small subset of TMs that a neighboring cell uses forall the PDSCHs. As an example, the serving eNB sends, in its RRCconfiguration message, a list of neighboring cell IDs and TM(s) to beused. Note that even a specific TM is indicated, current LTE allows a“fallback” override of the TM to a default T×D transmission format.Typically, this happens when an eNB finds previous PDSCH are notreliably received. Once the TM or TM subset is known, the UE can notonly reduce the complexity but also increase the robustness of channelestimation and presence/absence detection. For example, the UE only needto detect if DMRS is present or not if it knows that the interferencePDSCH can only be TM9 or a “fallback” to T×D. The eNB can furtherindicate no fallback if it wishes.

Another example is that TM also implies the rank of transmission, suchas TM2 is the rank-1 special case of TM3 and TM6 is the rank-1 specialcase of TM4. Hence, such TM signaling removes any need of RI detectionand associated error. In the case of DMRS-based TM8/9/10, the antennaport to be occupied by interference PDSCH may also be notified to the UEvia higher layer messages, and similarly for the parameter n_(scid) thatis required to generate the DMRS sequence. In TM10, the so-called“virtual cell ID”, which is the parameter n_(ID) ^(DMRS,i) (for i=0or 1) used for generating the DMRS sequence can also be informed to theUE.

Interference Channel Estimation

A victim receiver may estimate the inference channel with the aid ofknown pilot signals from co-channel interference. For example, if thesignaled information is for the generation of DMRS pattern ofinterference, such information of reference signal is beneficial for (1)detection the existence of interference on each RB; and (2) estimationfor the channel of interference. In this DMRS-based case, as alreadymentioned, the precoding information of inference is not necessarybecause DMRS is precoded by the same precoder applied on interference'sPDSCH. If the signaled information is for the generation of CRS patternof interference, it is helpful to estimate the interference channel, butnot for the precoding information of interference. In this case,obtaining the precoding information may rely on further signaling orreceiver's capability to detect it.

FIG. 7 illustrate channel estimation for an interference channel.Interference channel estimation is straightforward from DMRS, but not soin CRS-based TMs. Suppose R₀ and R₁ are the RE-positions for CRS port 0and port 1 associated with the serving cell and I₀ and I₁ are theRE-positions for CRS port 0 and port 1 associated with a neighboringcell. For the victim UE, the CRS REs marked by R₀ and R₁ suffer fromprecoded interference from the data channel transmitted by neighboringcells. Since a UE cannot use I₀ and I₁ to detect the presence/absence orthe PMI of the interference signal, the UE must perform the interferencedetection at R₀ and R₁ (i.e., CRS REs) or at the data REs of the servingcell, for each PRB in one slot. If the UE further knows theconfiguration information of the basic resource-allocation unit of theneighboring cell, as will be clarified later, it can collect moreobservations at those REs across more PRBs to make the interferencedetection more reliable.

On the data REs, the received signal is the superposition ofcontributions from both the unknown desired symbol and the interferencePDSCH symbol. On the other hand, at the CRS REs of the serving cell, thesignal corresponding to the serving cell can be estimated and subtractedto get the interference signal for later detection of the PMI used bythe interference PDSCH. There are several methods to detect the PMI usedby the neighboring cell. One approach is by comparing the observed LLRunder different hypotheses of PMI candidates at these CRS REs. Anotherapproach is by comparing the empirical covariance matrix of theinterference signal with that of the asymptotic one under different PMIhypotheses. Since the precoder used by neighboring cell usually is thesame over more than one PRB, one key to improve the detectionperformance is to know the location of resource-blocks where the sametransmission parameters are applied by the neighboring cell.

Resource Allocation (RA) Information

From the above discussion, it is noted that RA information can be veryhelpful for estimating the interference channel and for canceling theinterference PDSCH transmission. In LTE, the PDCCH contains all the RAinformation including DVRB or LVRB flag and the number of PRB allocated,PMI/RI for TM4&6, and modulation and coding scheme. Therefore, the UEmust detect those parameters on a PRB basis without the RA information.Note that there are only 4 CRS in each PRB (8 in a PRB pair) for eachCRS port, but a UE can use all the CRS in the entire system bandwidth.In CRS-based TMs, the UE must then detect those parameters from the dataREs or CRS REs within a PRB. In DMRS-based TMs, the UE can only use theDMRS in each PRB pair (i.e., 12 DMRS). To increase the robustness ofdetection, some RA information, if made available to the UE, cansignificantly improve the interference detection and cancellationperformance.

In one embodiment, the UE obtained the following configuration relatedto resource allocation of the interference PDSCH: an indication ofresource allocation type. In LTE, the most scheduling-flexible approachto signal resource-allocation is to signal the bitmap with a size thesame as the number of resource blocks within the cell bandwidth.However, this way may result in significant overhead especially when thecell-bandwidth is large. To reduce overhead, grouping of contiguousresource blocks is supported in DCI format 1, 2, 2A, 2B, 2C, and 2D fortype-0 resource allocation. A serving cell only needs to signal thebitmap of resource-block groups (RBGs) with reduced signaling overheadinstead of signaling for each resource block. In current LTEspecifications, DCI format 1 is used for TM1, TM2, and TM7; DCI formats2 and 2A are used for two CRS-based TMs: TM4 and TM3; DCI formats 2B,2C, and 2D are used for the following three DMRS-based TMs: TM8, TM9,and TM10.

Among the current standardized DCI formats, for DCI format 1 (for TM1,TM2, and TM7), DCI format 2 (for TM4), 2A (for TM3), 2B (for TM8), 2C(for TM9), and 2D (for TM10), there is a one-bit field to be signaledfrom serving cell so that a UE can know either type-0 or type-1RB-allocation is used. If type-1 RB-allocation is used, the resourceallocation is per-RB based with additional constraints. If the UElearned that the neighboring cell is restricted to type 0, it can assumethat transmission parameters including RI, PMI, MOD, and thepresence/absence of the interference are the same for the physicalresource-block “pair” in two slots in one subframe. Such an assumptioncan improve the detection reliability of those transmission parametersused by a neighboring cell. If not so indicated, a UE must do per-RBbased detection for transmission parameters of interference.

RBG-based signaling implies that if one of the resource blocks within aRBG is scheduled, the rest of resource blocks within the same RBG mustbe also assigned to the same user. In LTE, the RI and MOD must be thesame for all of the resource-blocks assigned to a UE. For CRS-based TMsexcept for TM2 and TM3, the used precoder must be same for all RBswithin one RBG. Thus, a UE may combine its observations on multiple RBswith same transmission parameters of interference to detect RI/PMI/MODinstead of applying per-RB-based detection.

In another embodiment, the UE can obtain the configuration informationof PRB bundling as the basic unit for resource allocation. PRB bundlingis already supported in DMRS-based TM9 when the UE is also requested toreport PMI/RI. However, such bundling operation can be madecell-specific to improve the parameter detection of interference. Inthat case, a similar higher-layer RRC message can be used to allow theUE to always use all the DMRS in the basic unit for detection. A similarmethod is to enforce subband based resource allocation when theallocation is constrained to be a subband. The basic unit for RA may bedirectly configured, instead of deriving from the information of TMs andsystem bandwidth. It can be the RBG size or the subband size for CSIfeedback already defined in current LTE system.

In yet another embodiment, LVRB/DVRB resource allocation is alsosupported in LTE to provide frequency diversity. A field calledLVRB/DVRB assignment flag is defined in DCI format 1A (all TMs), 1B(TM6) and 1D (TM5, MU-MIMO). One bit is used for this field to signalLVRB/DVRB. In case of DVRB and when the cell bandwidth is greater thanor equal to 50 PRB, another 1-bit is used to indicate either N_(gap1) orN_(gap2) is used.

For LVRB, there is no problem to let a victim UE to assume theRI/PMI/MOD is the same for the physical resource-block pair in two slotsin one subframe. For DRVB-mode, the index of physical resource-block fortwo slots corresponding to the same VRB index is split by a gap value,N_(gap). It is possible in theory to detect DVRB versus LVRB by, forexample, checking if some transmission parameters of interference, e.g.,power, MOD or PMI, are invariant across contiguous multiple physicalresource blocks. However, the reliability is doubtful considering thatDVRB is applied in a UE specific and dynamic manner. It is more feasiblewhen there is a higher-layer message to indicate that DVRB is applied ina cell.

In DVRB case, a UE still needs two more parameters to exactly know thephysical resource-block pair occupied by the interference PDSCH: thebandwidth size and the gap value N_(gap). As discussed earlier, themapping from VRB to PRB for two slots consists of one interleaving stepand one hopping step. After knowing the exact interleaving rule andhopping rule of the neighboring cell, the UE can then use a both PRB inthe two slots for interference channel estimation for example.

Currently in LTE, there are eight values (denoted as ρ_(A)) allowed forthe ratio of PDSCH EPRE (Energy Per Resource Element) to CRS EPRE, fordata REs in the OFDM symbols that do not contain CRS. There are also 4different ρ_(B)/ρ_(A) values allowed where ρ_(B) denotes the PDSCH EPREto CRS EPRE ratio for data REs on OFDM symbols that contain CRS. Bothρ_(A) and ρ_(B)/ρ_(A) are signaled as a UE-specific value, which meansit can change depending on the UE being scheduled. In one embodiment,the UE can be notified by its serving cell a subset of ρ_(A) andρ_(B)/ρ_(A) that may be applied in a cell specific fashion. As a result,the UE can detect these setting more reliably.

Interference Cancellation Example

FIG. 8 illustrates a method of cancelling data transmission from aneighbor cell in a mobile communication network comprising a UE 801, aserving base station eNB 802, and a neighboring base station eNB 803. Instep 811, UE 801 establishes a radio resource control RRC connectionwith its serving eNB 802. In step 812, UE 801 receives configurationinformation via RRC signaling. The configuration information may includesemi-persistent information such as slot alignment, CP length, systembandwidth, transmission mode (TM), cell ID, MBSFN subframes, CRS antennaports, P_(A), P_(B), and other parameters related to desired radiosignals to be transmitted to UE 801. In step 813, UE 801 receivesdesired radio signals from eNB 802. UE 801 is able to decode the desiredradio signal based on additional PDSCH configuration that is dynamicallysignaled via PDCCH.

In addition to the desired radio signals, however, UE 801 may alsoreceive interfering radio signals from other neighboring cells. In theexample of FIG. 8, neighboring base stations communicate with each othervia X2 interface or via proprietary signaling across eNBs of the samevendors. For example, eNB 802 may receive certain configurationinformation from eNB 803, such as resource allocation used by eNB 803for its data transmission. Exchange of configuration information betweeneNB 802 and eNB 803 could be via X2 interface 821. In step 822, eNB 802determines a second configuration information related to datatransmission from eNB 803, including slot alignment, CP length, systembandwidth, transmission mode (TM), cell ID, MBSFN subframes, CRS antennaports, P_(A), P_(B), etc. of the data transmission from eNB 803.

In step 831, eNB 802 sends the second configuration information to UE801 via RRC signaling. The second information may includesemi-persistent information such as transmission mode (TM), cell ID,MBSFN subframes, CRS antenna ports, P_(A), P_(B), and other parametersrelated to interfering radio signals to be transmitted to UE 801 fromeNB 803. In step 832, UE 801 determines a set of data resource elementsbased on the second configuration information. In one example, thesecond configuration information indicates that eNB 803 may adoptDMRS-based transmission modes, e.g., TM8 and TM9. Since DMRS are nottransmitted in PRBs when there is no PDSCH in that PRB, UE 801 maydetect the presence of DRMS-based PDSCH interference from eNB 803 bydetecting the presence of DMRS from eNB 803 at the data resourceelements corresponding to port 7 and port 8. In another example, thesecond configuration information may include resource allocation typeand basic resource allocation unit, which allows UE to assume the sametransmit parameters, e.g., precoder or modulation order, are used at theset of data resource elements within the basic resource allocation unitconsisting of multiple PRBs.

In step 833, UE 801 receives both desired radio signal from serving eNB802 and interfering radio signal from neighboring eNB 803 on the set ofdata resource elements. In step 841, UE 801 estimates the interferencechannel based on the received radio signals on the set of data resourceelements. In one example, UE 801 first estimates the desired signalcorresponding to a reference signal transmitted from the serving cell,and then subtracts from the received signals the estimated desiredsignal and thereby estimating the interference channel. In one example,the interference signal is obtained for later detection of the PMI usedby eNB 803. Note that the PMI detection performance can be improved whenUE 801 knows the location of the resource blocks where the sametransmission parameters are used by eNB 803. In step 842, uponestimating the interference channel, UE 801 is able to cancel theinterference signal of the data transmission from neighboring eNB 803.

FIG. 9 is a flow chart of a method of interference cancellation from UEperspective in accordance with one novel aspect. In step 901, a UEobtains configuration information of a data transmission in a mobilecommunication network. The data transmission is transmitted from aneighboring cell to the UE via an interference channel. The UE mayobtain such configuration information via high layer signaling from aserving cell. In step 902, the UE receives radio signals on a set ofdata resource elements as determined based on the obtained configurationinformation. In step 903, the UE estimates the interference channelcorresponding to the data transmission from the neighboring cell basedon the received radio signals on the set of data resource elements. Instep 904, the UE cancels the data transmission from the neighboring cellbased on the estimated interference channel. In one example, the UEcancels the data transmission based on the obtained configurationinformation, e.g., detecting PMI of the interference signal based onresource allocation information and cancel the data transmission fromthe neighboring cell accordingly.

FIG. 10 is a flow chart of one embodiment of a method of interferencecancellation from eNB perspective in accordance with one novel aspect.In step 1001, a serving base station transmits a first configurationinformation to a UE, the first configuration information is related to adesired signal of a data transmission from a serving cell to the UE. Instep 1002, the serving base station determines a second configurationinformation related to an interference signal of a data transmissionfrom a neighboring cell to the UE. The second configuration informationcomprises a resource allocation type of the interference signal. In step1003, the serving base station transmits the second configurationinformation to the UE such that the UE can cancel the data transmissionfrom the neighboring cell.

FIG. 11 is a flow chart of another embodiment of a method ofinterference cancellation from eNB perspective in accordance with onenovel aspect. In step 1001, a serving base station transmits a firstconfiguration information to a UE, the first configuration informationis related to a desired signal of a data transmission from a servingcell to the UE. In step 1002, the serving base station determines asecond configuration information related to an interference signal of adata transmission from a neighboring cell to the UE. The secondconfiguration information comprises a basic resource allocation unit ofthe interference signal. In step 1003, the serving base stationtransmits the second configuration information to the UE such that theUE can cancel the data transmission from the neighboring cell.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

What is claimed is:
 1. A method, comprising: (a) obtaining configurationinformation of a data transmission by a user equipment (UE) in a mobilecommunication network, wherein the data transmission is transmitted froma neighboring cell to the UE via an interference channel; (b) receivingradio signals on a set of resource elements (REs) as determined based onthe obtained configuration information; (c) estimating the interferencechannel corresponding to the data transmission from the neighboring cellbased on the received radio signals on the set of resource elements; and(d) cancelling the data transmission from the neighboring cell based onthe estimated interference channel.
 2. The method of claim 1, whereinthe configuration information includes a specific transmission mode or asubset of transmission modes used by the neighboring cell for the datatransmission.
 3. The method of claim 1, wherein the configurationinformation includes a resource allocation type used by the neighboringcell for the data transmission.
 4. The method of claim 3, wherein theresource allocation type includes a type 0 or type 1 of LTE downlinkresource allocation.
 5. The method of claim 1, wherein the configurationinformation includes a basic unit of physical resource blocks (PRBs)used by the neighboring cell for the data transmission.
 6. The method ofclaim 5, wherein the UE estimates the interference channel by applyingthe same transmission parameters for the basic unit of PRBs.
 7. Themethod of claim 1, wherein the configuration information includes abasic unit of consecutive physical resource blocks (PRBs) used by theneighboring cell for the data transmission using a localized type ofvirtual resource block (LVRB).
 8. The method of claim 1, wherein theconfiguration information includes an indication of a localized type ofvirtual resource block (LVRB).
 9. The method of claim 1, wherein theconfiguration information includes an indication of a distributed typeof virtual resource block (DVRB).
 10. The method of claim 9, wherein theconfiguration information further includes a system bandwidth and a gapvalue used to map the DVRB to a physical resource block (PRB) index. 11.The method of claim 1, wherein the existence of the configurationinformation indicates that the neighboring cell and the serving cell aresynchronized.
 12. The method of claim 1, wherein the UE cancels the datatransmission in (d) based on the configuration information obtained in(a).
 13. A method, comprising: (a) obtaining configuration informationof a data transmission by a user equipment (UE) in a mobilecommunication network, wherein the data transmission is transmitted froma neighboring cell to the UE via an interference channel; (b) receivingradio signals on a set of resource elements (REs) as determined based onthe configuration information, wherein the receiving of the radiosignals involves; determining a location of the set of REs based onresource allocation information derived from the configurationinformation; (c) estimating the interference channel corresponding tothe data transmission from the neighboring cell based on the receivedradio signals on the set of resource elements; and (d) cancelling thedata transmission from the neighboring cell based on the estimatedinterference channel.
 14. The method of claim 13, wherein the locationof the set of REs corresponds to reference signal transmissions of theserving cell.
 15. The method of claim 13, wherein the location of theset of REs corresponds to data transmissions of the serving cell.
 16. Amethod, comprising: (a) obtaining configuration information of a datatransmission by a user equipment (UE) in a mobile communication network,wherein the data transmission is transmitted from a neighboring cell tothe UE via an interference channel; (b) receiving radio signals on a setof resource elements (REs) as determined based on the obtainedconfiguration information; (c) estimating the interference channelcorresponding to the data transmission from the neighboring cell basedon the received radio signals on the set of resource elements; and (d)cancelling the data transmission from the neighboring cell based on theestimated interference channel, wherein (c) further involves: estimatinga desired signal corresponding to a reference signal transmitted fromthe serving cell; and subtracting from the received signals theestimated desired signal and thereby estimating the interferencechannel.
 17. A user equipment (UE), comprising: a configuration modulethat obtains configuration information of a data transmission in amobile communication network, wherein the data transmission istransmitted from a neighboring cell to the UE via an interferencechannel; a receiver that receives radio signals on a set of resourceelements (REs) as determined based on the obtained configurationinformation; a channel estimation module that estimates the interferencechannel corresponding to the data transmission from the neighboring cellbased on the received radio signals on the set of data resourceelements; and an interference cancellation module that cancels the datatransmission from the neighboring cell based on the estimatedinterference channel.
 18. The UE of claim 17, wherein the configurationinformation includes a resource allocation type used by the neighboringcell for the data transmission.
 19. The UE of claim 18, wherein theresource allocation type includes a type 0 or type 1 of LTE downlinkresource allocation.
 20. The UE of claim 17, wherein the configurationinformation includes a basic unit of physical resource blocks (PRBs)used by the neighboring cell for the data transmission.
 21. The UE ofclaim 20, wherein the UE estimates the interference channel by applyingthe same transmission parameters for the basic unit of PRBs.
 22. The UEof claim 17, wherein the configuration information includes a basic unitof consecutive physical resource blocks (PRBs) used by the neighboringcell for the data transmission using a localized type of virtualresource block (LVRB).
 23. The UE of claim 17, wherein the UE estimatesa desired signal corresponding to a reference signal transmitted fromthe serving cell and then subtracts from the received signals theestimated desired signal to estimate the interference channel.
 24. Auser equipment (UE), comprising: a configuration module that obtainsconfiguration information of a data transmission in a mobilecommunication network, wherein the data transmission is transmitted froma neighboring cell to the UE via an interfering channel; a receiver thatreceives radio signals on a set of resource elements (REs) as determinedbased on the obtained configuration information; a channel estimationmodule that estimates the interference channel corresponding to the datatransmission from the neighboring cell based on the received radiosignals on the set of data resource elements; and an interferencecancellation module that cancels the data transmission from theneighboring cell based on the estimated interference channel, whereinthe UE determines the location of the set of REs based on resourceallocation information derived from the configuration information.